HP 4540s Sporadically Wont Boot

[KingCornWallis]

I am at a severe loss in trying to figure out what is wrong with this laptop, so I am going to detail the laptops entire story:

Background

I purchased an 'As-Is' non-working HP 4540s with AMD Graphics on eBay a while back because it had a powerful i7-3720QM CPU I wanted to take out of it. When I got it however, I was surprised that it worked completely fine. I still took the processor out of it, and put a weaker i5-3210M CPU in...and it ran well for a while. One day, it refused to boot.

Problem

All the lights would turn on, the screen would be black, and the fans would run at full power for about 5 seconds, then it would shut off.

Troubleshooting

After much research I concluded that the CMOS battery must be bad, as any other problem should have given a beep code. After replacing the CMOS battery and still no display, I concluded the board must be bad.

So I swapped the board for another 4540s board with AMD Graphics. The same issue again. I put the old board back in and the old i7-3720QM CPU.

I have tried to fix this on and off for several years...and the weird part is, every few months that I try it, it will work fine. Until it doesn't.

Recently I tried the laptop again and found it booted to a black screen with fans on high, but it didn't shut off. I swapped out the i7-3720QM CPU for an i7 3610QM and lo and behold it worked again. Then it reverted to the old problem of shutting off shortly after turning on. Tried other RAM, tried other RAM slot, tried cleaning RAM, tried cleaning RAM Slots...No help.

Once I managed to isolate the issue to the drive; booting with the SSD plugged in caused the problem, and without it solved the problem. I tested this a few more times and the same problem returned.

I then removed ALL components so that the only things plugged into the motherboard were:

1. i7-3610QM CPU
2. 1x8GB PC3L-12800s RAM
3. CPU/GPU Cooler Fan
4. 1080p screen via LVDS Cable
5. Power Button via ribbon cable
6. CMOS Battery
7. HP 90W AC Adapter via Power Jack Cable

I power cycled the laptop and tried it, no luck.

I then removed the only RAM stick and tried again...not even any beeps, just brief high fan on black screen and shut down.

Questions

What in the WORLD could it be?

That small list of things plugged in was the same across both boards and producing the SAME problem!

Could a Defective screen cable, power jack, or or power button really cause this issue?

Anything helps... thanks